Mack-Cali could be adding to its Harborside complex in Jersey City. - (AARON HOUSTON)

Mack-Cali Realty Corporation is bolstering its Hudson Waterfront holdings in Jersey City, it said Monday, announcing plans to build Harborside 4, a 1.2 million-square-foot office tower, as part of a joint venture.

The Jersey City-based real estate investment trust said as part of its third-quarter leasing announcement that it is designing, leasing and building the latest Harborside building with Parsippany-based real estate developer SJP Properties.

“We chose SJP, who will be an equity partner in the proposed venture, because Steve Pozycki, the founder, is a master builder with an unmatched record of building iconic structures on time and on budget,” Michael DeMarco, Mack-Cali’s president, said in a prepared statement. “The project only gets built if we achieve substantial preleasing at our desired rates. We believe tenant demand is there today, as is our ability to finance and add an additional equity partner, if necessary. The Harborside 4 site is an existing parking lot that we own free and clear of debt for over 20 years.”

The REIT said it had executed 62 leases totaling 664,490 square feet of office and flex properties during its third quarter, it said. Of that total, 44 percent was new lease activity, while 56 percent represented lease renewals and other tenant transactions. Mack-Cali said its core, flex and Waterfront properties were at 90.3 percent leasing as of quarter-end, while overall leasing including non-core properties was at 87.7 percent, up 1 percentage point from the previous quarter.

“We are pleased with the significant leasing momentum we’re seeing across our portfolio, particularly along the Hudson River Waterfront, where occupancy achieved 95 percent,” Mitchell E. Rudin, CEO, said in a statement. “… We continue to be focused on core markets and key properties, with an eye toward transforming these assets with first-class amenity packages.

“Over the first nine months of this year, we have executed 218 leases totaling 2.5 million square feet. These results include 830,000 square feet of new leases and over 1.6 million square feet of renewal and other tenant transactions.”

Earlier in the month, Mack-Cali detailed its capital markets activity, announcing it has sold about $465 million in assets year-to-date, with another $265 million worth of dispositions under contract.

The company said it expects total dispositions for the year to reach about $730 million, including 27 office buildings comprising 4.5 million square feet, plus one residential building.

The company acquired one building in third quarter, it said, paying $235 million for 111 River St. in Hoboken, and it is currently under contract to acquire an additional $27 million portfolio.

In addition, Roseland Residential Trust, Mack-Cali’s multifamily subsidiary, said its third-quarter activity included breaking ground on the Lofts at 40 Park project in Morristown, a joint venture apartment community. Roseland began leasing at the M2 at Marbella project in Jersey City in May and, by the end of the third quarter, had reached 76 percent in commitments, it said.